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    <script>
        var arr = [1,2,3,4];

        var sum = arr.reduce(function(pre,item,index,arr){
            console.log('上一次return值或初始值',pre);
            console.log('当前遍历值',item);
            console.log('下标',index);
            console.log('原数组',arr);
            return item+pre;
        },5)

        console.log(sum);

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